The use of spectrophotometry has been applied to a wide range of materials to determine properties by measuring the adsorbance of radiation across a sample in a fluid cell. In such fluid cells one or more radiation transparent optical windows allow radiation to pass through the fluid sample contained in the cell and to pass through a detector. In the most common flow cell arrangements, radiation passes through an entrance window, a fluid sample and an exit window on its way to a detector as the fluid sample flows through the cell. The flow cells typically include a lens to focus radiation on the window in a desired pattern. The direction of radiation transmission and detection may be parallel to the flow of fluid through the cell or radiation and fluid may have a crossflow arrangement. Parallel and cross-flow arrangements for cells are disclosed in U.S. Pat. Nos. 4,192,614 and 5,078,493 the contents of which are hereby incorporated by reference.
Two important considerations in the arrangement of flow cells are the prevention of sample leakage around the radiation transparent window and control of the flow path length for the radiation across the sample. Leakage detection is particularly important when monitoring hazardous or corrosive fluids. Corrosive fluids pose additional difficulties on the operation of flow cells since any leakage of such fluids may impair the operability of the flow cell. U.S. Pat. No. 5,078,493 provides an O-ring between the corrosive environment of the fluid sample and the remainder of the flow cell containing the fiber optic cable lens and a sapphire window. The O-ring is made of materials that can resist the attack of highly corrosive substances such as hydrogen fluoride, hydrogen chloride or strong caustics. This arrangement uses a single O-ring in an axial or crush design wherein axial displacement of the sapphire window through the O-ring provides pressure for the O-ring seal. This arrangement provides a sealing element that can resist highly corrosive atmospheres and immediately indicates when a seal failure has occurred. Leakage of sample fluid between the lens and window or fiber cable and lens can interfere with the operation of the flow cell device and yield faulty measurements. Accordingly, there is a need for a flow cell having a seal arrangement that performs reliably and immediately indicate when any failure of the seal said has occurred.
In addition, there is a need for a flow cell that is easily manufactured and assembled while yet providing precise control of the radiation flow path length across the fluid sample. The axial or crush design of the seal disclosed in U.S. Pat. No. 5,078,493 interferes with the control of the radiation path length across the sample since the amount of pressure exerted on the O-ring will vary the displacement of the window relative to the sample.
It is an object of this invention to provide a flow cell arrangement that maintains seal integrity while warning of potential leakage.
It is a further object of this invention to provide a flow cell arrangement that improves control of the radiation path length across a fluid sample.